1. Field of the Invention
The present invention relates to medical ultrasound devices having transmitting transducers for beaming ultrasound waves into the body and adjacent receiving transducers for detecting the ultrasound waves reflected from internal body matter. More specifically, the present invention relates to ultrasound focusing mechanism for maximizing the reflected waves detected by the receiving transducer which waves were reflected from matter within a focal zone located a desired distance from the transducers.
2. Prior Art
In a medical Doppler ultrasound device, a transmitting transducer beams ultrasound waves of a known frequency into the body. A receiving transducer closely adjacent and stationary relative to the transmitting transducer detects waves reflected from internal body matter. The object is to detect waves reflected from matter, such as blood corpuscles, moving toward or away from the side-by-side transducers. In accordance with the well-known Doppler effect, the difference in frequency of a wave before and after reflection indicates the velocity of the matter from which the wave was reflected relative to the wave transmitter. A major use for medical Doppler ultrasound devices is to detect average blood velocity or, with the use of a frequency spectrum analyzer, the range of instantaneous velocities through a blood vessel. For such Doppler devices, only received waves having frequencies different from the frequency of the transmitted waves are of interest.
In medical "scanning" ultrasound devices, the object is to reflect transmitted ultrasound waves off stationary or substantially stationary internal body parts, to detect the reflected waves and to process a corresponding electrical signal to provide an image of the internal body part in plan or in section. Because the frequency of waves reflected from stationary or substantially stationary body parts will be the same or about the same as the frequency of the transmitted waves, only waves having a frequency close to the transmission frequency are of interest.
In a scanning device, the transmitted beam of ultrasound waves must be narrow in the area of the body part being scanned for a high quality image to be generated. A concave epoxy lens has been used to reduce the divergence of the transmitted beam and assist in creating a "zone of maximum intensity" or "focal zone" within which the beam is sufficiently narrow. The length and location of the focal zone also depends on the sizes of the transducers because only small transducers can be used to transmit beams that are narrow in an area close beneath the skin of the patient. Further, the transmission frequency affects the length and location of the focal zone because high frequency beams are less divergent but, unfortunately, attenuate more rapidly than low frequency beams.
Different transducers are used in the scanning devices depending on the depth of the body part being scanned. A transducer 6 millimeters in diameter and having a transmission frequency of 5 to 7.5 megahertz is typical of those used for scanning body parts 1 to 3 centimeters below the skin; a transducer 10 to 13 millimeters in diameter and having a transmission frequency of 3.5 megahertz is typical of those used for scanning body parts 4 to 10 centimeters below the skin; and a transducer 19 millimeters in diameter and having a transmission frequency of 2.25 megahertz is typical of those used for scanning body parts 7 to 19 centimeters below the skin.
Up until the present invention, focusing lenses had not been used in the medical Doppler ultrasound devices. Rather, different transducers were used depending on the depth of the blood vessel for which velocity information was desired, which usually is in the range of about 10 millimeters to 60 millimeters below the skin. In general, the transducers used for deep blood vessels were larger and of lower frequency than the transducers used for superficial vessels.
The required ability to accept a variety of transducers of different frequencies complicates the remainder of the medical Doppler ultrasound device. In addition, the output of the device is often an audible signal from a loudspeaker or headphones and the pitch and pattern of the signal is interpreted by the operating technician. Changing from a transducer of one frequency to a transducer of a different frequency changes the pitch of the output and can complicate interpretation of the output.